But you need the "Media Creation Tool", because only then you can control, when you download and update your machines.

Three machines, three woes

I personally have three eligible machines. One with Windows 10 Insider Preview, one with Windows 8, one with Windows 7. There was exactly none, where the update process went smoothly.

Windows 8.1 -> Windows 10 just not happening

One machine, a windows 8.1 laptop I have left to update itself without the "Media Creation Tool". It says its ready for Win 10 but the update never happens. This is going on since the release of Windows 10. I am just patient here, because I don't care that much.

29.9: Ok I finally after a month gave up, and said wuauctl/updatenow. Left it to install overnight. Came back to find it had failed with an error code that suggested I search for it (80240020). The top hit, with 600 likes, suggested that I delete all files in C:\WINDOWS\SoftwareDistribution. There was one file, that didn't want to go, so I rebooted. Now Windows installed some update, rebooted and crashed needing repair. Fortunately advanced options, repair was able to repair. Now back to wuauctl/updatenow...

30.9 (past midnight): Tried to update again. BAM Windows Update Error 8020056. Should I add, that there is nothing of value found in the Windows Update "more about this error"? Why yes certainly, as it's completely and utterly useless. Searching with google reveals, that I need to run DISM.exe /Online /Cleanup-image /Scanhealth. After maybe 30 minutes, it's done and I can now run Windows Update again, which will download 2.8 GB... Oops talked too soon, Windows Code 8024402C just happened. I can actually track this down to a Dota update saturating my extremely narrow broadband. Why doesn't it just say so ? Back to downloading... 1% right now.

30.9 (noon): It crashed once more again over night. Again with 80240020. This time I did not reboot after deleting C:\WINDOWS\SoftwareDistribution contents. Then I did wuauctl/updatenow again, and this times Windows 10, did download completly. I am now installing...

30.9 (afternoon): and it's up and running.

Windows 10 insider -> Windows 10, manual intervention required

One machine I could successfully (after the hang) migrate from Win 10 Insider to Win 10 regular, but not using the default GWX.exe you see in the task bar. That produced another inscrutable hex code error.

Instead the update process was like this: download GBs of Win 10. Run Installer, installer notices it needs another few GBs as Updates, downloads them. Installation went OK (hurray). Then Win 10 runs and download more GBs of Updates again ?? What gives ?

Windows 7 -> Windows 10, unbootable result

The last machine is a Windows 7 laptop. Windows 10 update crashed there with a blue-screen, taking the whole system with it. Now I have an unbootable laptop.